【BUG】 Failed during deploy if the branch of github in config is "master"

[leiqzhang]

If the "branch" name in config file is "master", then when deploy the project, an error message as follows will display before pushing posts to github successfully:

fatal: Cannot force update the current branch.

The bug exists in file lib/plugins/deployer/github.js：

var commands = [
          ['init'],
          ['add', '-A', '.'],
          ['commit', '-m', 'First commit'],
          ['branch', '-M', branch],
          ['remote', 'add', 'github', url]
        ];

Hexo deploy will run the commands above one by one. If the "branch" variable in the fourth command is "master", then the git command really run is:

git branch -M master

But the current branch is master already, so the message mentioned begin will be thrown and abort the deploy process.